Prevent socket selector thread from being suspended

[davidbrochart]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

When using debugpy to debug a Python program, one might want the AnyIO socket selector thread (on Windows with the ProactorEventLoop) to keep running when the program is stopped on a breakpoint.

Use case

It's the case in ipykernel where I currently have to write something like that:

if backend == "asyncio" and sys.platform == "win32":
    import asyncio

    policy = asyncio.get_event_loop_policy()
    if policy.__class__.__name__ == "WindowsProactorEventLoopPolicy":
        from anyio._core._asyncio_selector_thread import get_selector
        selector = get_selector()
        selector._thread.pydev_do_not_trace = True

The key part is selector._thread.pydev_do_not_trace = True, that ensures the thread is not suspended.
Do you think there should be a better API for that, or should it even be done by default?